Towards the identification of siderite, rhodochrosite, and vivianite in sediments by their low-temperature magnetic properties T. Frederichs a, * , T. von Dobeneck a,b , U. Bleil a , M.J. Dekkers b a Department of Geosciences, University of Bremen, P.O. Box 330 440, D-28334 Bremen, Germany b Paleomagnetic Laboratory Fort Hoofddijk, Utrecht University, Budapestlaan 17, 3584 CD, Utrecht, The Netherlands Abstract Siderite (FeCO 3 ), rhodochrosite (MnCO 3 ), and vivianite ([Fe 3 (PO 4 ) 2 ]  8H 2 O) are well-known authigenic minerals in a number of sedimentary settings. Here, we explore the potential of low-temperature mineral magnetic techniques for their identification at low concentration in bulk samples thus expanding mineral magnetic proxies for environmental purposes. The basic rock magnetic properties of these minerals, which are paramagnetic at ambient temperature, were determined with a ÔMagnetic Properties Mea- surement System’. Well-crystalline chemically analyzed material of natural origin was used to gather these data. The diagnostic value of the observed specific magnetic properties was tested on two mid-Eocene sediment samples from the Norwegian Sea (ODP Leg 104, Site 643) known to contain these minerals. The observed N eel temperatures of siderite (37 K) and rhodochrosite (34 K) conform with literature data. Both carbonates show a fairly strong spin-canted remanence (0.4 Am 2 /kg) from cooling in a 5 T magnetic field. Different ratios of field-cooled and zero- field-cooled remanences, however, allow a discrimination between the two minerals. A characteristic of rhodochrosite is its ex- tremely high magnetic susceptibility just below the N eel temperature. An almost vertical slope in very low fields of the hysteresis loop also testifies to this high susceptibility. It is assigned to a weak anisotropic ferromagnetism confined to the basal plane in which the spontaneous magnetization can almost freely rotate. A prominent magnetic property of siderite is its metamagnetism, resulting in a progressively upward bending of the hysteresis curve in magnetic fields above 5 T. Vivianite also shows an onset of meta- magnetic transition below 5 K in 5 T fields and a Ôtwo-stage’ increase in susceptibility between 2 and 12 K attributed to successive short- and long-range magnetic ordering. The magnetic properties of the two authigenic marine sediments could be largely explained by combining characteristics of rhodochrosite and manganosiderite. Shifting of the N eel points to lower temperatures and less well pronounced magnetic phenomena are attributed to element substitution and non-stoichiometry which occur commonly in sedi- mentary environments. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Siderite; Rhodochrosite; Vivianite; Mineral magnetism; Low-temperature magnetism; Metamagnetism 1. Introduction In rock and environmental magnetism the magnetic properties of rocks, sediments, and soils are physically investigated to explain their provenance and genesis. Variations in concentration, mineralogy, and grain size serve as proxy parameters in manifold applications, e.g., in paleoclimate or source tracing studies (Frederichs et al., 1999; Maher and Thompson, 1999). In general, these studies refer to minerals that are ferri- or anti- ferromagnetic at room temperature, i.e., to minerals that are able to carry a remanent magnetization under ambient conditions. These Ômagnetic minerals’ include iron oxides, sulfides and oxyhydroxides. Their specific properties allow a discrimination and quantification by magnetic bulk sample measurements (Peters, 1995; Pe- ters and Thompson, 1999; Peters and Dekkers, 2003). In the course of sediment diagenesis, various sec- ondary iron and manganese minerals form which are in most cases paramagnetic at room temperature. Among these authigenic minerals are the iron carbonate siderite (FeCO 3 ), the manganese carbonate rhodochrosite (MnCO 3 ), and the hydrated iron phosphate vivianite ([Fe 3 (PO 4 ) 2 ]  8H 2 O). They are typical of anoxic envi- ronments and indicative for geochemical conditions where ferric iron oxides usually dissolve (Canfield et al., 1992). Rhodochrosite is formed in both anoxic sulfi- dic and non-sulfidic environments whereas siderite is * Corresponding author. Tel.: +49-421-218-3989; fax: +49-421-218- 7008. E-mail address: thofred@uni-bremen.de (T. Frederichs). 1474-7065/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1474-7065(03)00121-9 Physics and Chemistry of the Earth 28 (2003) 669–679 www.elsevier.com/locate/pce